Characteristics and performance of a CO₂ heat pump at different refrigerant charges: An experimental study.

The charge amount (Mr) has a critical impact on the performance of CO₂ heat pumps, and the appropriateness of the charge is closely related to the operating parameters of the system. In this study, characteristics and performance of a CO₂ heat pump at different Mr levels has been investigated systematically based on an existing experimental bench. The results show that: (1) Analysis of evaporation temperature (Tev,r,in) based on ambient temperature can be a good indicator to reflect the system charge levels, and there exists an appropriate charge range for the system according to the operating conditions; (2) Mr is the dominant factor in determining Tev,r,in, while ambient temperature (Tam) acts as a ceiling that determines the upper limit of Tev,r,in. A mismatch between Mr and Tam can have a serious negative impact on the system performance; (3) Increasing the heating temperature (Th) can lower the charge demand of the system, and there exists a peak COPh corresponding to a specific Th. In the study ranges, the peak COPh values of 2.812, 2.475 and 2.204 are achieved at Th levels of 80, 90 and 100 ◦C, respectively; (4) The introduction of IHX leads to notable enhancements in both Tev,r,in and COPh. At Th levels of 80, 90 and 100 ◦C, the COPh of the system with IHX shows increases of 3.70 %, 6.05 % and 8.27 % respectively in comparison with to system without IHX.